Schmid et al
(Lancet, February 7) claim that "the major mode of spread of HIV-1" in Africa is
sexual in adults and mother-to-child transmission (MTCT) in children. Although
their evidence is consistent with such claims it cannot be considered proof.

MTCT should be
approximately the same in both sexes. Yet according to Fig.1 in children up to
the age of 4 years transmission appears restricted to girls.

In support of
sexual transmission they cite epidemiological evidence of the existence of a
parallel between HIV-1 and herpes simplex virus type 2 (HSV-2) seropositivity,
the age distribution of HIV-1 seropositivity and concordant seropositivity
amongst couples.

A parallel age
distribution between HIV-1 and HSV-2 seropositivity does not prove that HIV-1
like HSV-2 is sexually transmitted. Sexual behaviour intervention reduces
incidence of HSV-2, acute syphilis, gonorrhoea, and unprotected casual sex but
has no effect on HIV incidence. This means that the mode of HIV-1 acquisition
is not the same as HSV-2.1.
In fact no correlation exists between the prevalence of HIV and any of the STDs.2
Biggar et al3
were the first to report the age distribution of HIV-1 seropositivity. They
also showed that "The age curve for the prevalence of antibody against" HIV-1
parallelled that of antibodies against P. falciparum, a non sexually
transmitted microbe. This means that from the age distribution no conclusions
can be drawn regarding the route of transmission. Similarly, concordances among
couples does not prove sexual acquisition. Sexual partners exhibit concordances
for several common cancers which are not sexually transmitted.

Sexual
transmission can be proven epidemiologically only by following infected
individuals and their negative partners in prospective studies in which the
possibility of HIV acquisition by other means is excluded. There are many such
studies both in gay and heterosexual partners but for some unknown reason Schmid
et al did not mention any of them. All these studies show that like
pregnancy, a positive antibody test can be sexually acquired but not sexually
transmitted.4-6The
difference is that while pregnancy can be acquired by a single act of sexual
intercourse, for AIDS to appear a very high frequency of receptive anal
intercourse over a long period is necessary.

In the best
prospective study in heterosexuals Padian et al found no seroconversions
even after 6 years of follow-up.7.
In the largest study in African heterosexuals the authors concluded: "The
probability of HIV transmission per sex act in Uganda is comparable to that in
other populations…", that is, comparable to that found by Padian et al in
their cross-sectional study, very low.8
In other words, there is no more heterosexual transmission of HIV in Africa than
anywhere else including the USA, Australia and Europe. This means that
alternative explanations must be found for the exceedingly high seropositivity
reported from Africa.